Method for improving the long term stability of biodiesel

ABSTRACT

A method for improvement of the long term stability of biodiesel is described, in which the crude ester produced by transesterification of a vegetable or animal fat or oil with methanol, with a strong acid or with a mixture of a strong acid and a complex former is subsequently treated intensively and the ester layer separated from the emulsion formed thereby is subjected to a thorough water washing and subsequently dried.

The object of the invention is a method for the manufacture of improvedbiodiesel from fats and oils, especially from oil seeds, from which theoil is obtained by pressing or by solvent extraction, as well as fromYellow Grease.

Biodiesel is known to be produced by transesterification of untreatedglycerin esters, in Europe above all from rapeseed oil, by use ofmethanol and the addition either of acids (p-toluenesulfonic acid) orbasic catalysts, such as for example KOH, NaOH or sodium methylate. Inthat connection batch processes as well as continuous processes areutilized.

The vegetable and animal crude oils employed for the production ofbiodiesel always contain a proportion of free fatty acids and mucilages,which in the case of base-catalyzed transesterification lead to a highercatalyst consumption by saponification of the free fatty acids which arethen removed by way of the glycerin phase. The mucilages in the crudeoil are converted by direct transesterification into stable emulsions,which have a negative effect on the separation of ester and glycerinphases. Therefore preferably de-acidified and de-mucilated oils with afree fatty acids content of <0.1% and a phosphorus content of <20 ppmare utilized for the production of biodiesel. An especially suitablemethod for the conversion of a vegetable or animal oil or fat withmethanol or ethanol and an alkaline catalyst is described in GermanPatent Application 41 23 928, in which one operates with at least twoconversion stages, wherein each conversion stage contains a mixedreactor and a separator for separation of a light ester-rich phase froma heavy glycerin-rich phase.

After the transesterification has taken place, the residual glycerideand the dissolved glycerin as well as the salts and the saponified fattyacids are extracted from the crude methyl ester. For this purpose thecrude methyl ester is subjected to an acid treatment and subsequentwater wash by use of centrifuging, stirrer vessels or water columnsoperating on the counter current principle. The fatty acid methyl esterproduced thereby normally corresponds to the requirements of standardbiodiesel.

Though it has been shown that storage of the pure biodiesel afterwashing and drying fulfills the analytical criteria of standardbiodiesel, dependent on the fat or oil employed subsequent flocculationcan occur, independently of whether the biodiesel is additionallyfiltered or centrifuged after drying. 97% of these flocculations consistof organic material with an ash content of 3%, which is made up ofsingle or multivalent cations, sulfur and phosphorus compounds. Theseflocculations can deeply impair the usefulness of biodiesel as a fuel,because they lead rapidly to plugging of the fuel filter. It istherefore a critical prerequisite for the technical utility of biodieselthat also for longer storage the occurrence of flocculations beprevented with certainty.

In as much as it has been found that the occurrence of flocculations isbased on condensation- or crystallization-nuclei such as phosphatidesand ionic complexes of organic or inorganic nature, which are alwayscontained in the conventionally manufactured biodiesel, the object isset of markedly lowering the quantity of crystallization nuclei bywashing of the crude methyl ester so that subsequent flocculation is nolonger observed in the dried ester, on storage.

The invention relates therefore to a method for improvement of the longterm stability of biodiesel, in which the crude methyl ester produced bytransesterification of a vegetable or animal fat or oil with methanol,is intensively post-treated with a strong acid or with a mixture of astrong acid and a complex former, and the ester layer separated from theemulsion layer formed thereby is subjected to a thorough water washingand is subsequently dried.

Subsequent treatment of the biodiesel suitably involves the use of amechanically intensive mixer at temperatures between 25 and 60° C.

In accordance with the invention, hydrochloric acid, sulfuric acid orphosphoric acid are employed above all as strong acids, and as complexformer EDTA or citric acid.

The invention is based on the recognition that the flakes subsequentlyfalling out of the pure biodiesel are organic in nature and consist ofmaterials which are normally dissolved in the fatty acid methylester andtherefore also in biodiesel. This can include salts of long chain fattyacids, waxes or residual mucilage material. Following subsequenttransesterification these dissolved compounds are no longer separablemechanically by application of centrifuges and filters in the processingof biodiesel. Also dissolved organic compounds which are present in suchsmall quantities as to be barely detectable analytically can not betransferred with certainty from the ester phase into the aqueous phaseby washing of the crude methyl ester with centrifuges or wash columns.Only in the presence of a “crystallization nucleus” onto which thedissolved, and in the case of the ester finely divided, compounds can betaken up, it comes eventually to the point of visible flake formation.

The aim of the method in accordance with the invention for improvementof the long term stability of biodiesel can thus not be to remove theactual dissolved organic compounds taken up into the ester phase, but toeliminate the remnants with regard to “crystallization nuclei”. Thesecan be single or multivalent cations, residual soaps from fatty acidshaving multivalent cations, complexes of inorganic or organic compoundsor phosphatides, or mucilage.

The removal of crystallization nuclei is achieved on the one hand by amore intensive washing process and on the other hand by an additionalhydrophilization of the crystallization nuclei in order to make themmore easily separated from the organic ester phase in the subsequentwater wash, also at the same time an intensified splitting of the longchain calcium and magnesium soaps as well as to bring about the definiteremoval of iron compounds.

The method in accordance with the invention is explained in furtherdetail by means of the appended FIG. 1.

The crude ester formed by transesterification of a vegetable or animalfat or oil or a Yellow Grease with methanol is initially present in amixture with the split-off glycerin. This mixture contains asimpurities, un-reacted methanol, soaps, free glycerin, mono anddiglycerides, residual alkaline catalyst (for example sodium methylate)as well as phosphatide residues (mucilages) and organic and inorganiciron compounds from the fat or oil employed. In the Settler 2, the heavyphase containing the glycerin and residual catalyst is separated and fedback into reactor 1 employed for the transesterification.

The crude ester phase is on the other hand fed to an intensive mixingapparatus (Ultra-Turrax Inline Mixer) for splitting of the soaps with astrong acid or with a mixture of a strong acid and a complex former suchas for example citric acid or EDTA. With an energy input of 0.002 kW/kgof crude ester, a fine emulsion is produced at about 50° C. in thespecial mixing chamber of the Inline Mixer from the ester and from theacid phase, which is fed to Settler 3.

After a waiting period of about 30 minutes a heavy phase separates fromthe ester and the emulsion breaks. A stable interphase develops betweenthe ester phase and the heavy phase which is not separable into theheavy phase even with application of a commercial centrifuge.

The thus pre-purified ester is then fed to a subsequent water wash. Thiswater wash may be carried out in a stirrer vessel or a centrifuge or ina wash column operated on the counter current principle. Quiteespecially an intensive washing with the Ultra Turrax Inline Mixer hasproved effective.

The purified biodiesel thus produced is practically free of anycondensation and crystallization nuclei as well as mucilage materialsand iron compounds and has such outstanding long term stability thateven after drying the biodiesel to remove residual traces of water, nocloudiness or flocculation occurs upon long term storage.

It is surprising moreover that with the method in accordance with theinvention the utilization of an intensive mixer leads in the shortesttime to splitting of the soaps by strong acids, and furthermore that inpresence of strong acid, alcohol and water the phosphatides and ironcompounds which are still present in the ester dissolve out of theorganic phase and are carried over into the heavy aqueous phase and intothe interphase. For the separation of these impurities, the combinationof an intensive mixer with a downstream settler has proved to beexcellent, whereas the phosphatide containing interphase layer was notdefinitively separated from the ester by utilization of a centrifuge.

By means of the method in accordance with the invention the long termstability of biodiesel was successfully increased considerably in anextremely simple way, so that cloudiness and flake formation can beentirely avoided, and thereby the up until now extremely damagingplugging of pores and filters can be forestalled in the utilization ofbiodiesel.

1. Method for improving the long term stability of biodiesel, wherein1.1 crude methyl ester is formed by transesterification of a vegetableor animal fat or oil with methanol, 1.2 the crude methyl ester thusformed is intensively post-treated at temperatures between 25 and 60 C,with a strong acid or with a mixture of a strong acid and a complexformer characterized in that 1.3 a fine emulsion is prepared from theester and the acid phase, 1.4 the ester layer separated from theemulsion formed thereby is subjected to a thorough water wash andsubsequently dried.
 2. The method according to claim 1, characterized inthat, as strong acid hydrochloric acid, sulfuric acid, ptoluenesulfonicacid or phosphoric acid are employed and as complex former EDTA orcitric acid.
 3. The method according to claim 1, characterized in thatthe water wash of biodiesel is carried out in a wash column according tothe counter current principle or by means of a mechanically intensivemixer.